The invention relates to a pressure swing adsorption process for dual recovery of a more readily adsorbable component, such as nitrogen, and a less readily adsorbable component, such as oxygen, from a feed gas mixture in which the copurge:feed gas ratio of the process is less than about 1.15:1 so that the ratio of the more readily adsorbable component (nitrogen) to the less readily adsorbable component (oxygen) is less than about 3:1.
In numerous chemical processing, refinery, metal productions and other industrial applications, high purity oxygen or nitrogen may be needed. Enriched oxygen gas or nitrogen gas is frequently required for metal treating atmospheres and other applications. Nitrogen and oxygen gases can, of course, be obtained by various known techniques for air separation. Pressure swing adsorption (PSA) processing is particularly suited for such air separation in a variety of applications, particularly in relatively small sized operations for which the use of a cryogenic air separation plant may not be economically feasible.
The most common PSA systems produce a single enriched purity gas stream, usually the less readily adsorbable (light) component, from a given feed supply. In these systems, a feed gas mixture is passed through an adsorbent bed capable of selecting the more readily adsorbable (heavy) component at a higher pressure. The light component passes through the bed and is collected as product. The heavy component is then desorbed from the adsorbent at low pressure and exhausted from the system as waste. PSA systems are designed differently when the primary product of interest is the heavy component. In such systems, a co-current displacement purge, consisting of product quality gas, is passed through the bed in the direction of the feed following the feed step. The light component passes through the bed and is exhausted at the discharge end of the bed as waste. High purity heavy product is collected at the feed end of the bed through a series of blowdown, evacuation and purge steps. In either of the prior art systems, only a single component of the feed mixture is captured as product, while the remaining components are exhausted from the system as waste. In such cases, the waste stream is generally not sufficiently enriched in a desired component for use in a chemical or industrial application. In each of these systems, the unit cost of product is determined relative to the single product of interest, either the heavy or the light product.
U.S. Pat. No. 4,599,094 describes an air separation process in which the primary product is high purity nitrogen, but from which a reasonably high purity coproduct oxygen is also produced. This process uses a high-pressure ratio (ratio of maximum adsorption pressure to minimum desorption pressure) of ten or greater in combination with 13xc3x97 molecular sieve. High nitrogen recovery is a major objective. Product ratios (N2:O2) much greater than 3:1, and even approaching the theoretical maximum of 4:1, are achievable with this process. Product nitrogen purities xe2x89xa799.8% (with recovery xe2x89xa798%) and oxygen purities of 90% to 93.6% were demonstrated. A chief reason for such high recoveries and purities is that the oxygen/air mass transfer front, discharged in the adsorption step, is partitioned and completely refluxed to other beds in the process. This complexity and the high-pressure ratio generate high capital and operation costs resulting in expensive products.
U.S. Pat. No. 4,810,265 teaches a co-current displacement process for the production of nitrogen. Feed air is introduced into the inlet of the bed, nitrogen is selectively adsorbed and oxygen enriched gas is withdrawn from the discharge end of the bed. The air feed step is followed by a co-current purge (co-purge) displacement step in which high purity nitrogen is fed into the bottom of the bed while oxygen enriched gas continues to be withdrawn from the top. The nitrogen purge gas displaces the oxygen that had been co-adsorbed with nitrogen, as well as the oxygen remaining in the interparticle void space. The co-purge flow continues until the mass transfer front erupts from the discharge end of the bed and the oxygen purity degrades. The oxygen-rich (light component) gas is discharged from the top of the bed and is either vented as waste or collected for countercurrent purge and/or pressurization. The adsorbent and void spaces are then saturated with high purity heavy product. U.S. Pat. No. 4,810,265 also teaches the use of LiX or 13xc3x97 adsorbents in the above-described process.
U.S. Pat. No. 4,013,429 relates to pressure swing adsorption process in which ambient air, during an on-stream period, is passed serially through a pretreatment adsorbent bed removing moisture and carbon dioxide therefrom. The dried and purified air is then passed through a main adsorbent bed selective for retention of nitrogen, the oxygen-rich effluent being collected in an expandable receiving vessel. The pretreatment and main adsorbents are contained in separate vessels connected in series. Nitrogen of high purity is desorbed by evacuation from the main bed in a direction opposite to that of the initial air charge. This nitrogen product passes from the main bed into and through the pretreatment bed to a collection vessel. Preceding the vacuum desorption step both the pretreatment bed and the main bed are rinsed with the high purity nitrogen product gas from a previous stage in the operation. Following evacuation, the beds are repressured with a portion of the oxygen-rich gas drawn from the expandable receiving vessel. By operation in the described manner there are recovered for any desired use, nitrogen of high purity (99.7 to 99.9%) and oxygen-enriched (78 to 90%) gas product. Recovery of both nitrogen and oxygen products is about 95%. Such high recovery of both products is indicative of prior art processes seeking to optimize the N2:O2 product ratio as near to 4:1 as possible. The process of U.S. Pat. No. 4,013,429 is both complex and expensive, having sixteen cycle steps, thirteen valves, four compression machines and three expandable gas receivers.
U.S. Pat. No. 4,892,565 relates to a process for recovery of a more selectively adsorbed key component from a gas mixture containing the key component and one or more less selectively adsorbed secondary components using vacuum swing adsorption. The process minimizes capital costs by reducing or eliminating gas storage vessels and reduces power requirements by operating without a feed compressor, whereby feed is introduced at least in part by vacuum conditions achieved by pressure equalization between parallel adsorption beds. The major thrust of this invention is the production of the more selectively adsorbed component at relatively high purity (xe2x89xa795%). Alternatively, at least a minor amount of less selectively adsorbed secondary component product can be recovered. Clearly for air separation, this three or four-bed process is directed at the recovery of the heavy nitrogen component alone, or alternatively at a high N2:O2 product ratio.
U.S. Pat. No. 4,915,711 relates to an adsorptive separation process set forth for recovery of two gas products in high recovery and high purity using adsorption, depressurization, low pressure purge, evacuation and repressurization. Depressurization and purge effluents are recycled to feed. Optionally, pressure equalizations are performed after the adsorption and after the evacuation steps. This invention is aimed at CO2/CH4 separation. In terms of an air separation option, only processes utilizing an oxygen-selective adsorbent are considered, i.e. the oxygen is the heavy component. The process also differs markedly from the present invention in the use of the co-current depressurization step following the adsorption step and in the use of the low-pressure purge of the heavy component.
The prior art has been primarily driven to produce high purity nitrogen (99.9% or greater) and to maximize the yield of nitrogen from such air separation processes. Oxygen has been recovered from such processes as an additional benefit to offset some of the costs of producing the high purity nitrogen. A perfect or ideal separation would yield nitrogen and oxygen products in a 4:1 ratio. In real processes, such as disclosed in U.S. Pat. No. 4,810,265 and U.S. Pat. No. 5,163,978, the recovery of the primary nitrogen product is maximized. The resulting product ratio (N2:O2) in such processes is well above 3:1. Thus, the prior art has apparently found that minimum nitrogen unit product cost corresponds to maximizing the N2:O2 product ratio to values greater than 3:1. A practical upper limit of this product ratio, which is necessarily less than the ideal ratio of 4:1, is the consequence of mass transfer resistances and the co-adsorption of nitrogen and oxygen in the mass transfer zone. In other words, it is impossible to remove all of the oxygen from the bed without losing a portion of the nitrogen.
Contrary to the PSA co-products air separation prior art, wherein maximizing the N2:O2 product ratio is desirable, the subject invention seeks N2:O2 product ratios less than 3:1. One aspect of the subject invention is that the light oxygen product amount can be increased if the co-purge:feed air ratio is decreased. Instead of simply venting excess high purity nitrogen product to obtain a lower product ratio, the novel process of the present invention reduces the co-purge:feed air ratio (e.g. by approximately 10%) to achieve a greater amount of light product and a corresponding lower N2:O2 product ratio. This results in a reduction in the unit cost of oxygen.
The utilization of an inlet blower, which provides the dual function of a feed air blower and a co-purge compressor, can be changed so that more feed air and less co-purge nitrogen are introduced into the adsorber bed. Thus, higher feed throughput and greater light component oxygen product are obtained using the same fixed-size equipment.
It is an object of the invention to provide a PSA process and system for the recovery of a more readily adsorbable gas component along with a less readily adsorbable gas component from a feed gas mixture wherein the amount of the less readily adsorbable (oxygen) gas is increased by regulating the ratio of the co-purge:feed gas to less than 1.15:1. Consequently, the ratio of the more readily adsorbable gas component to the less readily adsorbable gas component is decreased to less than 3:1.
It is another object of the subject invention to provide a PSA process and system capable of minimizing the capital costs, power consumption, and overall costs of recovering a greater amount of enriched oxygen product from air.
With these and other objects in mind, the subject invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.
The invention relates to a pressure swing adsorption/desorption process for the co-production of a more readily adsorbable gas component (such as nitrogen) and a less readily adsorbable gas component (such as oxygen) from a feed gas mixture comprising the feeding of the feed gas into at least one adsorbent bed containing an adsorbent material for pretreating or removing undesirable components and then through a main adsorbent bed. The feed gas mixture passes through the main adsorbent at elevated pressure wherein the heavy component is adsorbed and the light component exits as product. This step is followed in sequence by a co-current purge (co-purge in the same flow direction through the main adsorbent bed as the feed) of the high purity heavy product. Heavy product is recovered in subsequent countercurrent blowdown and evacuation steps. The main bed is then countercurrently purged and then repressurized with light product. Such cycle steps are carried out in a manner to produce a product ratio of the more readily adsorbable gas compound to the less readily adsorbable gas component of less than about 3:1, preferably about 2:1, wherein the co-purge:feed gas ratio is regulated to less than about 1.15:1, preferably less than 1.05:1.
When the feed mixture is preferably air, a representative process of the invention can be described as follows: (a) introducing the less readily adsorbable component (oxygen) into the discharge end of the bed to partially countercurrently repressurize the bed from a subatmospheric desorption pressure level to an intermediate pressure level, such countercurrent backfilling of the bed serving to displace any previously adsorbed more readily adsorbable gas component (nitrogen) toward the feed end of the bed;
(b) passing the feed gas mixture to the feed end of the bed, the bed being co-currently repressurized from said intermediate pressure to an upper adsorption pressure, with said more readily adsorbable gas component being selectively adsorbed and said less readily adsorbable gas component being selectively withdrawn from the discharge end of the bed in which a larger amount of less readily adsorbable gas is produced by maintaining a co-purge:feed gas ratio of less than about 1.15 in the process, a portion of said larger amount of the less readily adsorbable gas component being withdrawn from the system for other applications, the remaining portion of said less readily adsorbable gas component being passed directly to another bed in the system for said countercurrent backfilling repressurization of step (a) or for the purging of the bed and/or being passed to a surge tank for subsequent use in such backfilling or purge steps;
(c) passing the more readily adsorbable gas component to the feed end of the bed at said upper adsorption pressure so as to co-currently purge said bed with a co-purge:feed gas ratio of less than about 1:15:1, preferably less than about 1.05, and said less readily adsorbable gas component being withdrawn from the discharge end of the bed so that the bed is cleaned out of the less readily adsorbable gas component by said purge prior to the recovery of said more readily adsorbable gas component from the bed;
(d) countercurrently depressurizing the bed by discharging said more readily adsorbable gas component from the feed end of the bed, the pressure of the bed thereby being reduced first from the upper adsorption pressure to atmospheric pressure through blowdown and then to a subatmospheric desorption pressure level preferably through the use of a vacuum pump;
(e) countercurrently purging the bed at subatmospheric pressure by introducing said less readily adsorbable gas component to the discharge end of the bed and said more readily adsorbable gas component being discharged from the feed end of the bed preferably through a vacuum pump, thereby increasing the adsorptive capacity of the bed prior to the next succeeding pressurization adsorption step, and said more readily adsorbable gas either withdrawn from the system or stored in a surge tank for use as purge gas in the process; and
(f) repeating steps (a)-(e) on a cycle basis with additional feed gas being passed to the bed during the carrying out of the process to maintain a co-purge:feed gas ratio to less than about 1.15:1 and thereby maintaining the ratio of said readily adsorbable component to said less readily adsorbable component gas to less than about 3:1.
If the less readily adsorbable component is oxygen and the more readily adsorbable component is nitrogen, then said oxygen could preferably have a purity of above about 60%, more preferably above about 80%, and said nitrogen could preferably have a purity of above about 98%, more preferably above about 99.8%; and such gases being conveniently obtained as low cost products in the simplified processing cycle of the invention.
The processing cycle of the invention generally comprises various pressurization and depressurization steps operating between a low (subatmospheric) desorption pressure and an upper (above-atmospheric) adsorption pressure, coupled with advantageous purge or displacement steps at said upper and lower pressures to enhance the recovery of the less readily adsorbable gas component recovered at the high desorption pressure and the recovery of the more readily adsorbable gas component at the lower desorption pressure. Various processing modifications can also be employed in particular embodiments to enhance the performance of the process and system of the invention as applied with respect to the requirements of particular air separation or other feed gas separation applications.
The subject invention is preferably practiced in a PSA system for the separation of oxygen and nitrogen from air wherein at least two adsorbent beds are employed, with each of the beds undergoing the processing cycle herein disclosed in an appropriate sequence as related to the other beds in the system so as to facilitate the carrying out of continuous gas separation operations in such a system. In preferred embodiments of the invention, two or three adsorbent beds are commonly employed.
It has been discovered, unexpectedly, that the oxygen product amount can be increased if the co-purge:feed gas ratio is decreased. Instead of simply venting excess high purity nitrogen product to obtain a lower product ratio, the novel process produces a system than can reduce the co-purge:feed air ratio to less than 1.15:1, preferably less than about 1.05:1. This will achieve a greater amount of the light product oxygen and thereby a lower nitrogen to oxygen ratio (less than about 3:1). This will also result in a reduction in the unit cost of oxygen.
The reduction in the co-purge:feed air ratio is accomplished by reducing the amount of time for co-purge and increasing the amount of time for air feed. Adsorbent efficiency can be further improved by monitoring and using the purity of the oxygen-rich gas discharged from the top of the bed to control the co-purge:feed air ratio. This novel process will maximize the amount of oxygen produced from a fixed amount of adsorbent material.